1. Field of the Invention
The present invention relates generally to an automatic transmission primarily intended for motor vehicle use, and more particularly, to a hydraulic circuit for controlling the lock-up clutch of a torque converter assembly in an automatic transmission.
2. Description of the Related Art
Automatic transmissions are designed to take automatic control of the frictional units, gear ratio selection and gear shifting. In general, the major components featured in such an automatic transmission are: a torque converter assembly; fluid pressure-operated multi-plate drive or brake clutches and/or brake bands which are connected to the individual elements of the planetary gearsets in order to perform gear shifts without interrupting the tractive power; one-way clutches in conjunction with the frictional units for optimization of power shifts; and transmission controls such as valves for applying and releasing elements to shift the gears (instant of shifting), for enabling power shifting, and for choosing the proper gear (shift point control), dependent on shift-program selection by the driver (selector lever), accelerator position, the engine condition and vehicle speed.
The control system of the automatic transmission is typically hydraulically operated through the use of several valves to direct and regulate the supply of pressure. This hydraulic pressure control will cause either the actuation or deactuation of the respective frictional units for effecting gear changes in the transmission. The valves used in the hydraulic circuit generally include spring-biased spool valves, spring-biased accumulators and ball check valves. Since many of these valves rely upon springs to provide a predetermined amount of force, it will be appreciated that each transmission design represents a finely tuned arrangement of interdependent valve components.
Typically, the hydraulic circuit includes a valve assembly to control the fluid pressure to the torque converter assembly. An example of such a valve assembly is disclosed in Leising et al. U.S. Pat. No. 4,875,391 which is assigned to the same assignee as the present application. While the patented valve assembly has worked well, one disadvantage is that the valve assembly uses two-sided control for the lock-up clutch of the torque converter assembly. Another disadvantage is that the patented valve assembly does not have fluid pressure feedback on the lock-up control valve. A further disadvantage is that the patented valve assembly lacks a structure to dampen or cushion the application of the lock-up control valve.